The present invention relates to an apparatus for reading a color image using a solid-state image pickup element or the like, and, more particularly, to a color image reading apparatus for guiding light from an object to a sensor such as a solid-state image pickup element through a focusing optical system and a color separation means.
As an apparatus for line-scanning an object such as an original in a sub-scanning direction and reading a color image using an array of solid-state image pickup elements (e.g., CCD sensors), an apparatus shown in FIG. 1 is known. In FIG. 1, information on a portion of an original illuminated with light from an illumination light source (not shown) is color-separated into three color components by a three-piece (3P) prism 20 through a focusing optical system 19. The three color components are then focused on and read by three 1-line CCD sensors 21, 22, and 23.
In this prior art, however, three independent sensors are required, and the 3P prism 20 must have high precision, resulting in high manufacturing cost. Furthermore, adjustment between focused light beams and the sensors 21, 22, and 23 is required at three different positions, resulting in great difficulty during manufacturing.
Three parallel lines of sensor arrays may be mounted on a single substrate to be separated by a finite distance, and three lines of sensors may be formed on one element as a monolithic three-line sensor.
Such a three-line sensor 24 is shown in FIG. 2A. In FIG. 2A, distances S.sub.1 and S.sub.2 between two adjacent lines of three lines 25, 26, and 27 are set to be, e.g., about 0.1 to 0.2 mm due to various manufacturing conditions. In FIG. 2B, dimension a and b of each unit element 28 are, e.g., about 7 .mu.m.times.7 .mu.m or 10 .mu.m.times.10 .mu.m.
FIG. 3 shows a known arrangement of a color image reading apparatus using the above-mentioned monolithic three-line sensor as a light-receiving element. In FIG. 3, when information on an original surface 18 is line-scanned and read in the sub-scanning direction, light reflected by the original surface 18 is color-separated into three color light components by color separation beam splitters 30 and 31 each having a dichroic selective transmission film through a focusing optical system 19, and these light components are focused on the corresponding sensor arrays 34, 35, and 36 on a monolithic 3-line sensor 32.
As shown in FIG. 3, however, when the thickness of each of the beam splitters 30 and 31 is represented by t, an interarray distance on the sensor 32 is given by 2.sqroot.2t. If the interarray distance (2.sqroot.2t) is set to be 0.1 to 0.2 mm, as described above, the thickness (t) is about 35 to 70 .mu.m. This numerical value does not allow easy manufacture in consideration of a required flatness or the like of a surface.
Furthermore, a color image reading apparatus using a blazed diffraction grating in place of a dichroic mirror is also known by U.S. Pat. No. 4,277,138 (corresponding to DE2645075). In this arrangement, an optical system using a blazed diffraction grating is provided as a color separation means.
With this arrangement, however, light from only one point of an object is taken into consideration, and no consideration is given to so-called field angle characteristics based on a finite read width in the main scanning direction which is present in an object surface.